1. Field of the Invention
This invention relates to a data loss prevention method of a media storage device which uses a head to record data on a magnetic disk or other recording media, and in particular relates to a data loss prevention method of a media storage device which prevents in advance data loss due to degradation of the recorded data on the recording media due to thermal relaxation, and to a media storage device.
2. Description of the Related Art
Demands imposed on electronic data processing in recent years have led to requests for media storage devices, such as magnetic disk devices and magneto-optic disc devices which store data in media, with increased data capacities. As a consequence, the track densities and recording densities of storage media continue to increase. There are also demands to reduce unused areas on tracks.
Information is recorded on magnetic media for data recording and storing through positive and negative magnetization inversions. In some cases, certain fluctuations, which may be magnetic noise and slight magnetization inversions due to atomic-level thermal energy, may cause magnetization in a direction to cancel this recording magnetization, so that the magnetization state is weakened, and the recorded coercive force is reduced. This phenomenon is called “thermal relaxation” or “thermal fluctuations”.
When the thermal relaxation phenomenon occurs, a reduction in S/N ratio, that is, worsening of the error rate tends to occur, and problems relating to reliability such as failure to read previously recorded data may arise. In the worst case, there is the possibility that data may be lost.
Such a decline in recording coercive force (decline in magnetization) has until recently occurred with a time constant of several tens of years or longer, and so until now has not been considered a problem at all; but with increases in recording densities in recent years, and time constants of the order of several years, the occurrence of declines in magnetization has come to be regarded as a problem.
For example, FIG. 20 and FIG. 21 are used to explain a case in which the error rate worsens with the passage of time due to thermal relaxation. As shown by the data in FIG. 20 for the BER (bit error rate) with the passage of time after writing, a case is considered in which the BER is degraded by the 0.53 power of ten, that is, 1×10^(−6.0) errors/bit to 1×10^(−5.47) errors/bit, from an elapsed time of 1 minute (in logarithmic representation, 1.78) to 60 minutes (in logarithmic representation, 3.56). As indicated in FIG. 21, represented as the ratio to the time elapsed from (as a logarithm) 1.78 (1 minute) to 3.56 (60 minutes), this is 0.53/(3.56−1.78)=0.53/1.78=0.3.
That is, there is a degradation of approximately 0.3 power per decade. If this is expressed as 0.3 power/decade, then after five years (8.2 decades), a degradation of 0.3× (8.2−1.78)=1.93 power occurs. Hence at time of product shipment, if for example the quality assurance period is 5 years, then assurance against the occurrence of unrecovered errors (bit error rate or BER=1e−13 (−13th power)) is necessary for as long as five years after data has been written. In this case, the product must be shipped after verifying the error rate with error rate degradation due to thermal relaxation over five years superposed on the error rate at the time of product shipment.
In this example, a product shipment system is necessary which is able to provide an assurance of a BER of −14.93 power, resulting from an unrecovered error occurrence assurance (BER=−13th power) on which is superposed the thermal relaxation-related degradation over 5 years of 1.93 power.
But when using such a method, because recording and reproduction characteristics differ depending on the head and recording media characteristics, in addition to employing heads and recording media with still better characteristics, during tests at the time of product shipment it is necessary to perform shipment tests in which the thermal relaxation degradation is measured and a playback margin added corresponding to the degradation forecast to occur over five years. These tests require time and labor, and are unsuitable for mass production.
In light of this, in the past the various methods described below have been proposed for preventing data loss due to thermal relaxation.
(1) Separately from ordinary data, reference data is recorded in a prescribed area of the recording media, and depending on the reproduction level of this reference data, the necessity for prevention of data loss for the corresponding data is judged (see for example Japanese Patent Laid-open No. 10-255202 (FIG. 4)).
(2) Because recorded data is degraded with the time elapsed from recording, the address and recording time of recorded information is stored in recorded information units, with the current time provided by the host is compared with the recorded time to determine the elapsed time, to judge the necessity of re-recording. In addition, changes in the temperature of the media are detected, and when changes are considerable, re-recording is performed (see for example Japanese Patent Laid-open No. 10-255209).
(3) Recorded data is reproduced, the reproduction level is compared with a reference level, and a judgment is made as to the occurrence of degradation due to thermal relaxation (see for example Japanese Patent Laid-open No. 2001-216605 (FIG. 3)).
Recently there have been demands for storage devices with higher recording densities and lower prices. The conventional technology (1) assumes that the reproduction level of the reference data is proportional to the level of the actual data; but as is well known, the thermal relaxation phenomenon differs depending on the recording pattern, and so it is difficult to accurately determine which data should be re-recorded by detecting the reproduction level of reference data. If large quantities of reference data are recorded in order to alleviate this problem, then a large prescribed area for reference data recording must be provided on the storage media, and the problem of unused recording areas arises.
In the case of the conventional technology (2), the reference signal and the reference signal recording time are recorded, and the quality of the reference signal is monitored; when degradation of the quality of the reference signal is detected, recorded information recorded at the same time as the recording time of the reference signal is re-recorded. Because the reference signal is a data series which is easily affected by thermal fluctuations, it differs from actual data series, and so there is the problem that data for which re-recording is not necessary is also judged to be in need of re-recording.
However, because recorded information units are for example sector units, numerous time information items must be stored, and moreover comparison of times requires time. And because judgments are made only as a function of temperature changes and elapsed time, recorded data for which there is no decline in quality is also re-recorded, so that there is a large possibility that a vast amount of unnecessary processing will be performed.
In the case of conventional technology (3), recorded data must be read out and a level judgment made, and when there is a vast amount of data recorded on the storage media, there is the problem that time is required to judge degradation; moreover, in order to detect the level of the reproduced data, apart from a read data channel, a separate special channel comprising an A/D converter is necessary, so that additional hardware is required.